TEM intro

Question 1

Why simply heating tungsten wire is not optimal source of electrons to obtain good phase contrast in TEM.

Answer 1

Because tungsten wire tip is quite wide. Thus the spatial coherence of the electron beam is quite poor and then imaging your sample through phase contrast becomes more challenging and inaccurate. On the other hand heating is required as it is a thermionic source. Thus the source will be expanded wwhich further meesses up coherence(also temporal).

Question 2

How does the strength of the C1 lens affect the probe

Answer 2

A stronger lens will lead to upper focus point. This will also be translated by C2 to a smaller probe on the sample. However, a lot of electron will be excluded at the C2 aperture because of the wider deflection of electrons. Thus the probe will also be dimmer.

Question 3

What consequence does the nature of the lorenz force have for your images?

Answer 3

they will be rotated. This is because the lorenz force generated by the magnetic field of the magnetic lenses, results in a spiraling motion through the lens field

Question 4

Role of soft iron core.

Answer 4

The soft iron core (or pole pieces) is used to concentrate/amplify and shape the magnetic field. Iron, being highly permeable, directs and intensifies the magnetic field lines into a strong, localized area within the bore of the lens. The gap between the pole pieces creates a narrow, concentrated magnetic field region where electron manipulation occurs.

Question 5

role of C1 condenser lens

Answer 5

The C1 lens determines the spot size of the source on the specimen

Question 6

role of C2 lens

Answer 6

*The C2 lens controls the illumination of the specimen

Question 7

Detective Quantum Efficiency (DQE)

Answer 7

The DQE of a camera determines the contrast and information that can be
extracted at any given spatial frequency. It represents the amount of transferred info and depends on sample frequency.

Question 8

Types of cameras

Answer 8

film
CCD
CMOS (direct) detectors

Question 9

CCD vs film

Answer 9

film actually has better DQE. CCDs are however more convenient.

Question 10

Apertures

Answer 10

The useful part of an electron lens is extremely small – only the centre tens of microns of an electron lens is any good at focussing electrons. For this reason, we have to use apertures. Smaller aperture gives higher contrast because more electrons are removed
from the beam
* The same effect can be used by using lower energy (= longer wavelength)

Question 11

CCD camera principle

Answer 11

scintilatr converts electron signal to light. Charge Coupled Device then detects the light ourput and records the signal. The reason for the photon mediator is because electrons can otherwise damage sensors. THis however is also why the photon transformation releases a lot of heat, which results in low SNR. Cooling devices are needed to acount for this.

Question 12

CMOS principle

Answer 12

Direct electron detection. complementary metal-oxide semiconductor.